Etching process



May 12, 1959 A. JONES ET AL ETCHING PROCESS Filed June 5, 1956 Elmmozuxm 2m:

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2,886,420 il .Patented .May .12, f1 959 ETCHING PROCESS Augustus Jones,Troy, and Fred J. Haskins, Rochester, N.Y., assignors to GeneralDynamics Corporation, Rochester, N.Y., a corporation of DelawareApplication June 5, 1956,'Serial No. 589,532

Claims. (Cl. 41-42) The present invention relates to etching baths, andmore particularly to a process and apparatus for regenerating an etchingbath in a manner to maintain its activity.

An object of the present invention is the provision of a generallyimproved and more satisfactory etching bath process.

Another object is the provision of a process and apparatus formaintaining the activity of an etching bath, which are simple andrelatively inexpensive.

Still another object is the provision of a bath regenerating -processwhich is particularly adapted and suitable for use with etching baths offerric chloride.

A further object is the provision of both a process and an apparatus forthe recovery of metal chloride as a valuable by-product produced in theetching of a metal by aferric chloride etching bath.

A still further object is the provision of a process and apparatusadapted especially but not exclusively for the etching of copper by aferrc chloride solution and for the recovery of copper chloride as aValuable by-product.

These and other desirable objects may be attained in the mannerdisclosed as an illustrative example of the .invention .in the followingdescription and in the accompanying drawing forming a part hereof, whichdrawing constitutes both a flow sheet illustrating a preferred exampleof the process of the invention and a diagrammatic illustration of apreferred form of vapparatus in accordance with the invention.

In certain types of etching operations, as for example when etchingmetals such as copper, tin, aluminum, zinc, and Imany others, and theiralloys, it is desirable to use an etching bath of ferrie chloride,because it is relatively f active for the intended purpose andrelatively safe with respect Vto the operating personnel. However, theactivity kof the etching bath decreases with a decrease in `the amountof ferrie chloride in the bath (the ferrc chloride being converted toferrous chloride during the etching` reaction) and with an increase inthe amount of metal or alloy etched away from the article being etched.As the concentration of chlorides of the etched metal or metals in thebath increases, and the concentration of ferrc chloride decreases, apoint is eventually reached at which the activity of the bath is soreduced that the use of the bath is no longer commercially economical.

Also, `in many cases it is not commercially economical to discard thebath when it reaches this point of exhaustion, and substitute a new orfresh bath, because the t ferrc chloride is relatively expensive(whether purchased either as crystals or as a previously preparedsolution) so that the substitution of a new and fresh bath or even theaddition of more ferrc chloride to the old .bath would involveundesirable vexpense of considerable amount. Moreover, there is thefurther factor that bythe time the Vetching bath vhas been vused longenough `to reduce its activit-yito a relatively low `level,.itmay-contain valuable metallic salts, which shouldnot be discarded if aninexipensive way =to recover them can be found.

The present invention serves not only to replenish or i2 rejuvenate theactivity of the etching :bath,xbut lso Ito recover the metallic salts inIa commercially economical manner. v

rAs a convenientillustrativeexample, theinventionwill be described withIreferen'ceztorthe etchingof` COPPrlaminate plates of the kind known :inthe electronics lindustry s printed circuit plates. These are'thinsheetsof copper laminated to`a base .sheet or panel'of electrical vinsulatingmaterial (e.'g., aephenolic'sheet ysuc'h'as Bakelite), certain portionsyof -the Icopper Ybeing covered, :before the etching yprocess begins,rby a resist -layer of known form. When the copper laminatesheet isimmersed in the etching bath, the bathfeats away those portions of thecopper which are not covered by the resist layer, leaving intact `thoseportions which'are coveredby the resist layer.

The etching of printed circuit -plates `furnishes a good example to forma basis -for describing the invention, 4because it illustratesrthatcertain factors, e.g., the tempera- .tureof the etching bath, maybe limited by or be dependent upon the characteristics of the insulatingsheet or the characteristics of the bond between the copper sheet vandthe insulating sheet. The'choice of this `illustrative example alsoenables the inclusion of the step of recovery of a valuable by-product,since the copper chloride formed'vduring 'the etching vreaction can berecovered andA sold to advantage. -But it must be clearly borne 1in'mind that copper printed'circuit plates, or 'indeed copper in any .shape:or form, is but an'ex-ample and is not intended, as a limitationuponthe scope orfusefulness of the invention. The 'invention is applicableequally well to the etching of any of the many metals which are capableof being attacked or etched by ferrie chloride, e.g., lead, tin, nickel,iron, cobalt, cadmium,

-chromium, zinc, aluminum, magnesium, or alloys of any such metals.The'term metal as hereafter used in this application is intended, unless-otherwise plainly indicated, yto include alloysas wellas pure orrelatively -pure metals. And where copper happens to be mentioned,asthedevscription proceeds, it will be understood that this metal isnamed only vfor the sake-of giving the desired specific example, andthat any Aother metal (including alloys) capablev lof being etched -byferrie chloride could be substituted, except as otherwise plainlyindicated, without departing from the invention.

Referring "now to the drawing, the etching equipment 1l is o-fconventional kind, usually embodying la tank for `containing the etchinglbath of ferrie chloride, provided with suitable racks 'or hangers forsupporting the work `to -be etched. The work may be bathed with ferricchloride either by mmersing the Work in the-solution or by `spray- `ingwith the solution. As already mentioned, the etching V-bath :used inaccordance with the .present invention is sists the corrosive actionofthe .etching bath. The same `is true 'of `all of the other containers,conduits, valves, pumps, jandother pieces of equipment which come intocontact with the etching solution.

.According .tothe presentinvention, vwhen etching metallic articles in abath vof vferrc chloride, Athe bath in the tank 11 is initially composed-essentially of ferrc chloride having, atrthe start, a specilicgravityin the neighborhood of 42 Baume, corresponding to 39 rpercentferrc chloride, when the .articles beingA etched `are composedfof cop-;per `or tof nickel or cobalt Yor other metals which .are `attackedby:the'iferric chloride at `jabout `the same :rate :as copper, or :havingiagsomewh-at ,lower `specilic gravity when the articles are of metalattacked more rapidly by ferric chloride, such as aluminum or magnesium.Such etching is continued, by successive removal of nished work andinsertion of fresh work tofbe etched. As etching continues, there is adrop in the concentration of the ferric chloride etching solution, thisdrop in concentration being caused by the reaction of the bath with themetal being etched, which converts part of the ferric chloride intoferrous chloride, accompanied by the formation of a chloride of themetal being etched (e.g., copper chloride when copper is being etched).In the prior practice, when the concentration of the bath is lowered toapproximately 24 percent ferric chloride (in the case of etching ofcopper) it is found that the use of the bath is no longer economical,because the required etching time becomes too great, and the bath isthen discarded and replaced by a fresh bath.

According to the present invention, as distinguished from the abovementioned prior practice, the bath is preferably not discarded in toto,but rather is treated (preferably in a continuous manner) both toregenerate the bath and to recover the chlorides of the etched metal,which constitute valuable and saleable by-products. To accomplish this,the present invention provides a flow (preferably continuous, but it maybe intermittent if desired) of active etching solution into thecontainer 11 and of partially spent solution out of the container. Theentering bath is of ferric chloride (carrying in solution a minor amountof chlorides of the metal being etched, after the process has been inoperation for some time, as explained below) at a specific gravity ofabout 42 Baum or close to it for example when etching copper or othermetals having similar etching characteristics, but lower when etchingaluminum, magnesium, or other fast-reacting metals and at a workingtemperature as high as can safely be used without damage to the workbeing processed or to the equipment. When etching copper laminate on aphenolic base, the limiting factor in the Working temperature of theetching bath is usually the danger of injury to the phenolic sheet or tothe bond between the copper and the insulating sheet, and the workingtemperature is preferably kept at about 120 F. lf a base sheet and bondhaving higher heat resistance are used, the working temperature can becorrespondingly raised, so long as it does not exceed the safetemperature with respect to the lining or other delicate parts of theapparatus. When etching an article entirely of metal, or a laminatedarticle in which neither the base sheet nor the bond constitutes alimiting factor, the temperature may be raised substantially to theboiling point of the etching solution, if the tanks, conduits, Valves,etc., will Vsafely stand this temperature.

In the etching bath, ferric chloride is converted to ferrous chlorideand (when copper is the metal being etched) copper chloride is formedfrom the copper eaten away. But the concentration of the bath is notallowed to go lower than about 24 percent ferric chloride. The

vpartially spent etching bath is drawn olf througha con- 'duit 13 andmetering valve 15, to iron addition apparatus 17 comprising, forexample, a liquid container having a few iron plates immersed therein sothat the bath may react with the iron plates to pick up enough iron tocompensate for the iron dragged out of the etching bath by loss of thatpart of the bath which clings to the work vwhen the finished etched workis withdrawn from the container 11. A by-pass 18, valved at 20, extendsaround the iron addition apparatus.

From the iron addition apparatus 17, the liquid passes through a conduit19 to the top of a chlorinating tower 21, which may be in the form of aconventional chemical reaction tower lled with ceramic packing ofconventional kind, down which the solution trickles to the bot- -tom ofthe tower, while in contact with an upward or counter-flow of chlorinegas introduced at the bottom of the tower at 23, which gas reacts withthe ferrous chloride in the descending Aliquid and converts at least amajor part of it and preferably all of it to ferric chloride. Anychlorine gas reaching the top of the tower is drawn off at 25 tochlorine recovery apparatus for reuse.

The chlorinating tower has no eiect upon the chloride of the metal beingetched, which was included along with ferric chloride and ferrouschloride in the liquid entering the top of the tower. Therefore, theliquid reaching the bottom of the tower consists essentially of amixture of chloride of the metal being etched and ferric chloride, withpossibly a small amount of unregenerated ferrous chloride, but inrelatively minor quantity, if any. The rate of outflow of the liquidfrom the etching bath 11 is so regulated with respect to the amount ofwork being etched in the bath, that this liquid withdrawn from .the bathand reaching the tower will contain chloride of the metal being etchedin a concentration close to but slightly below its saturation point in asolution of ferricferrous chloride, at the particular workingtemperature of the bath. The reaction in the tower produces heat, thusraising the temperature of the'liquid in the tower somewhat, so thatthere is no danger of premature crystallization in the reaction tower ofthe chloride of the metal being etched. Moreover, steam is preferablyadded to the tower at 27 to raise the temperature of the regeneratedsolution to 212 F., to free the solution of chlorine gas.

This liquid at the bottom of the tower, consisting essentially of ferricchloride carrying in solution a high concentration (but less thansaturation) of chloride of the metal being etched, and possibly minorquantities of ferrous chloride, is withdrawn from the bottom of thetower through a conduit 31 valved at 33, to a heat interchanger35, andpasses through the cooling portion 37 thereof which is in heatexchanging relation with a heating portion 39. After being partiallycooled in the heat interchanger, the liquid passes through a conduit 41to a cooler 43 where it is cooled to an extent sufficient to crystallizeout the major part of the chloride of the metal being etched, whichcrystallizes out in the form of a crude or unrened form of suchchloride. The exact temperature to which the liquid is cooled in thecooler 43 is not critical, so long as it is low enough to crystallizeout most of the metallic chloride (other than the more solubleferric-ferrous chloride), and the exact temperature will depend on theavailable supply of cooling water flowing through a cooling coil orother suitable cooling means associated with the coolerl 43. In mostlocalities, cooling water is available at temperatures not above about65 F., and cooling is usually carried to about this temperature.

The crystals of metallic chloride are removed from the cooler 43 at 45and are further treated in any suitable manner. For example, they may bedried in a centrifuge dryer, washed with water, dried again, andpackaged for sale. The liquid from the cooler 43, after separation ofthe etched metal chloride crystals therefrom, is drawn off through theconduit 47 and now consists of a relatively cold liquid solution offerric chloride, with a minor amount of etched metal chloride insolution therein and possibly a small amount of ferrous chloride. Theetched metal chloride will be in saturated solution, but its quantitywill be relatively small because of the low temperature of the solution.This cold solution is passed through the control valve 49 and conduit 51to the heating portion 39 of the heat interchanger 35, to be partiallywarmed by heat extracted during the passage of the warm solution throughthe cooling portion 37.

After this partial heating in the interchanger 35, the liquid flowsthrough the conduit 53 to a storage tank 55 in which it is heated by anysuitable means such as the heating jacket 57, back to the desiredworking temperature of the etching bath, which, as above explained, isgoverned by the ability of the equipment and the work being etched towithstand high temperatures, and which mayrordinarily be in theneighborhood of F., when f etching rlarriinated printed circuit plates,for example. The 'heating of the liquid' raised the solubility ofthelcontained etched metal chloride, so that the latter i'snow v vway belowthe saturation point. From this storagetank containing thenow'regeneratedbath' at ,the proper -ternperature, the liquid is fedthroughthe conduit 61 and regulating valve-63 intothe etching container11, where lit again comes into contact with the work ytolbe etched. IThis cycle isrepeated indeiini'tely. i

The description` thus far has proceeded on the assumption that theetching processhadbeenoperating for some time rbefore any regenerationorcrystallization, so that the etching process had before regeneration'and crystallizationdecreased the concentration ofthe ferrier chloridedownto below substantially 27 percent'and hadY built up Itis seenY thatin the'preferred form ofthe'- invention, the etchingv process canbecarried on continuouslyfinra very satisfactory, ecient, and economicalmanner. 170

a relatively high concentration of etchedy metal chloride 'in theetchingA bath, approaching the saturation point thereof atY thegworkingtemperature ofV the bath. When this is not the case (i.e., whensimultaneously starting both the regenerating-crystallization randetchingpioc` esses) it may be desired to regenerate thefferrouschloridethough the process'has Ynotyetfproduced enough' etched metal chlorideVto be crystallized out economically. Yv Vlien l is the case, the flowmay be divertedthrough thel conduit 65` andrcontrol valve `6K7, toby-.pass the heattinter- 'echanger 35 and cooler Y 43, thusreturningrtheregenerated liquid fromthe chlorinatingtower A21 directlyto the kstor-"r'agetank .55 without the recovery of crystals. Also, there maybettimeswhen it is desired to transfer liquid from the cooler r43direct tothestorage tank 55 without sending it ,through the heatinterhauser y35;,and this. may, be accomplished by a .byfpassconduit 7,1 valved at 73.

vOne or more pumpsj are provided wherever necessary to insure flowthrough the Vvarious conduits. Asanexamplerone pump has been indicatedat 75. in the. conduit `31 leadinsfromthe bottomof the chlorinatinstower Y2.1 to the heat interchanger 35. Water is added `at any dei siredpointin the ow circuit, to make up for the Water `V,taken out as waterof crystallization in .the crystallized `etched metal chloride. Forexample, the vmake-up Water may be added to the storage tank 55 througha` conduit j 81' valved at S3.

l AA number of variations are possible without-departing from theinvention. For example, the viron additionap- Y paraitus is notnecessarily located between the etchinggcontainer 11and thechlorinatingV tower 21, butmay'be placedv at any other desired point inlthepath of low; or instead of using separate iron addition apparatus,the iron plates may be physically placed in the chlorinatingtower nearthe top thereof, so that as the ferrie chloride inthe mixtureenteringthe tower passes over the iron plates, it! will kreacttherewith to Apickupv iron and form a fresh supply-y vof ferrous chloride, which then isconverted to ferrie chloride as it owsdown through the chlorinatingtower.,

Again, it is not necessary that the regenerating. or

chlorinating stepprecede the cooling step to crystallize out the etchedmetal chloride. The flow may extend direct from the etching equipment 11to the heat interchanger 35 and crystallizing cooler 43, and then Vtheregenerating or chlorinating step may take place in the Vcourse of theow from the cooler 43,to,the storage tank 55.V This has the advantagethat the heat produced by f .consisting essentially of ferrie chloride.with a relatively recapitulate briefly, the work to b'e etched, inthe'f'orm of articles wholly or partly of metal, is inserted andremoved.from the etching container Y11l from time to time. Through the conduit61, there is a constant inflow into the etching container 11 ofreplenishment liquid-consisting essentially of ferric chloride as itsactive ingredient, at a relatively lhigh concentration-of, say, 39percent, .and at a temperature of, say,vl20 VF., which replenishmentliquid contains a relatively low concentration of chloride of the metalbeing etched, far below-the saturation concentrationthereof atpthisworking temperature. There is a continuous outflow (inthe preferredcontinuouslprocess) o fliquid from the etching container. 11I throughthe conduit 13, the outowing liquid consisting essentially of a mixtureof ferric chloride and ferrous chloride at' a considerably reduced'Vconcentration of ferric chloride and carrying `in solution with it,etched metal chloride-at a relatively high concentration of the solutionat this particular temperature, which temperature is still close to' ifnot the same Vas the* initial working temperature yin the etchingequipment 11.

'lhis liquid mixture offerric chloride, ferrous chloride, and etched,-metall chloride isk then regenerated in the chlorinating tower. Theregenerating reaction or activity raises the temperature oftheregenerated'solution, and it is preferablyfurther raised by addition ofsteam to clear the solution of chlorine gas, so that there is no dangerof premature crystallization of the etchedmetal chloride at isthenpartially cooled inthe kinterchanger 35 and further v cooled intheLcooler 43 `to crystallize out the Aetched metal chloride, thecrystals of which may carry with them water of crystallization,whereupon the cooled mixture, now

minor amount of etched metal chloride, goes either directly orthroughthe heat interchanger 35 to the storage'tank 55 to be heated totheWorking temperature (e.g., 120 F.) and thence to ow back into theetching container' 11 for reuse.`

This process, in its preferred form, greatly reduces'the cost of etchingas compared with the prior practice of using batches of ferric chloridewhich have heretofore been discarded and wasted when no longerA iit foruse. The cost of the chlorine gas required for regenerating the ferrouschloride toferric chloride isvusually considerably less than the costofan equivalent amount of a new or fresh batchof ferric chloride purchasedaS such, in addition to Which there may bea substantialmarket value forthe recovered etched metal chloride crystals (particularly if the metalbeing` etched is copper) so that there is a double-.saving in cost, bothfrom the regeneration step and the metallic chloride recovery step.

It may be kmentioned here that the valves 15 and 20 are adjusted tofsuchextent as necessary to pick up enough iron from theiron additionapparatus so that the solution in the storage tank 55 is 'kept at aspecic gravity of about 42 Baume (when copper and similarly reactingmetals are. to beetched), which specific gravity corresponds to aconcentration of about 39 percent ferrie chloride by weight, in water.If the concentration rises above this,

If the cooling water supply has a temperature of 65 or cooler, thesolution is preferably cooled in the cooler 43 to a temperature of about75 F. Assuming that copper is the` metal being etched, the copperchloride formed in the etching process will have a solubility of about6.41

. moles per 1000 grams of water at the formation temperasolubility ofthe chloride of the etched metal, at the higher temperature in the bath11 as compared with the lower temperature in the cooler 43, it thusbeing advisable to operate the process with the bath 11 as hot as isreasonably possible under al1 the circumstances, and the cooler 43 ascold as is reasonably possible in view of the availability of cheapcooling water.

Iron was mentioned above, among the metals which could be etched by theferric chloride. However, when etching iron, the iron cannot berecovered by crystallization or precipitation. Therefore, when using thepresent invention in etching iron, the regenerating or chlorinating partof the process (tower 21, etc.) is employed just as when etching othermetals, e. g., copper, except that one starts with a relatively weaketching solution of ferrie chloride, e.g., about 27 Baume, correspondingto about 24 percent ferric chloridez After leaving the chlorinatingtower, the solution is sent back to the storage tank 55 through theby-pass 65, omitting the cooling and crystallizing step entirely, exceptas needed to place the solution entering the tank 55 at the propertemperature. The solution will ygradually become more and moreconcentrated as the process continues. When the concentration iinallyreaches the highest limit which may be safely used as a practicalmatter, a portion of this concentrated ferrie chloride solution iswithdrawn. Then, the remaining solution is restored to its originalconcentration and volume by the addition of water. The

withdrawn concentrated ferric chloride solutionrmay be sold.

Also, where the solubility of the chloride of the metal being etched isvery high (as is the case, e. g., with aluminum chloride and zincchloride) there may not be enough difference in solubility at theetching bath temperature and at the cooling temperature to make itpractical to crystallize out the chloride. In such cases, it may be morepractical and economical to omit the cooling step (except as may beneeded to bring the temperature down from the tower exit temperature ofabout 212 to the storage tank temperature of about 120) and to run theprocess until a highly concentrated ksolution of the etched metalchloride is produced, then discard the solution and start again with afresh batch. But in either case, the regenerating or chlorinating stepof the present invention is employed, and constitutes a substantialimprovement over the prior art even when the cooling and crystalrecovery step is not employed. Of course when etching metals whosechlorides have substantially diferent solubility at the diiferentworking temperatures, such as copper, nickel, and cobalt, both theregenerating step and the crystallizing-out step are employed incombination with each other, and this constitutes the preferred form ofthe invention in its full fruition.

It Will be apparent from the foregoing that by the proper choice of therelative capacities of the etching equipment as compared to theregenerating tower the solution in the etching bath may be kept at ornearly at lthe same concentration of ferric chloride as the new solutionwas, assuming drag out iron and evaporated water have been replaced.This is possible by selecting a tower having a capacity suflicientlylarge so that it will always be able to regenerate all the ferrouschloride that would be generated by the maximum capacity of the etchingequipment. lt is alsoobvious that as the capacity of the etchingequipment is increased the capacity of the crystallizing equipment wouldalso have to be increased in order to handle the increased volume ofmetallic salts to be precipitated. At this point, we Would like toemphasize that the exact means to etch the metal by the etchant forms nopart of this invention. The term placing metal to be etched in anetching bath as used in the claims is intended to cover not only bathingby u'nmersion, using either air or mechanical agitation, but alsospraying, splashing the etching solution on the work, or by stillimmersion whereby the etchant is brought in contact with the work.

' It is seen from the foregoing disclosure that the above mentionedobjects of the invention are well fulfilled. It is to be understood thatthe foregoing disclosure is given by way of illustrative example only,rather than by way of limitation,'and that without departing from theinvention, the details may be varied within the scope of the appendedclaims.

What is claimed is:

1. The continuous process of etching metal with ferrie chloride whichcomprises the steps of bringing metal to be etched into contact with aliquid etching bath the active ingredient of which is ferric chloride,to form in said bath a liquid mixture of ferric chloride, ferrouschloride, and chloride of the etched metal, continuing the etching untilthe concentration of etched metal chloride in said bath approaches thesaturation point at the working temperature of the bath, continuouslywithdrawing from the bath a quantity of said liquid containing saidconcentration of etched metal chloride, subjecting the withdrawn liquidto reaction with chlorine gas to convert at least a substantial part ofthe ferrous chloride thereinV to ferric chloride, cooling the withdrawnliquid to crystallize out a substantial part of the etched metalchloride, removing the etched metal chloride crystals from the cooledliquid, reheating the cooled liquid to the working temperature of theetching bath, and returning the regenerated liquid to the etching bath.

2. The continuous process of etching copper with ferric chloride whichcomprises the steps of bringing copper to be etched into Contact with aliquid etching bath, the active ingredient of which is ferrie chlorideat a temperature of substantially F. and at a concentration ofsubstantially 39 percent, to form in said bath a liquid mixture offerrie chloride, ferrous chloride, and cupric chloride, continuing theetching until the concentration of the bath drops below substantially 27percent and the concentration of cupric chloride in the bath approachesthe saturation point at the working temperature of the bath,continuously withdrawing from the bath a quantity of said liquid,subjecting the withdrawn liquid to reaction with chlorine gas to convertat least the major part of the ferrous chloride therein to ferricchloride, cooling the withd-rawn liquid to approximately 65 F. tocrystallize out a substantial part of the cupric chloride, removing thecupric chloride crystals from the cooled liquid, reheating the cooledliquid to substantially 120 F., and feeding the regenerated and reheatedliquid continuously to the etching bath.

3. The process which comprises the steps of placing metal to be etchedin an etching bath composed essentially of ferrie chloride and arelatively low concentration of chloride of the metal being etched, toform in said bath ferrous chloride and a relatively high concentrationof chloride of the metal being etched, withdrawing a portion of saidbath from the presence of the metal being etched, regenerating thewithdrawn portion by reaction with chlorine to convert the ferrouschloride therein to ferrie chloride, cooling the withdrawn portion tocrystallize out a part of the etched metal chloride, separating thecrystallized chloride from the cooled portion of the bath, reheating thecooled portion to the working temperature of the etching bath, andreturning the regenerated and reheated portion to the etching bath.

4. The process as defined in claim 3, further including the step ofadding iron to the withdrawn portion of the bath to replace iron lost inthe form of iron chloride clinging to an article which has been etchedwhen such article is removed from the etching bath.

5. The process which comprises the steps of placing metal to be etchedin an etching bath composed essentially of `ferrie chloride and arelatively low concentration Of chloride of the metal to be etched, toform in said bath ferrous chloride and a relatively high concentrationof etched metal chloride, withdrawing a portion of said bath from thepresence of the metal being etched, regenerating the Withdrawn portionby reaction with chlorine to convert the ferrous chloride therein toferrie chloride, and returning the regenerated portion to the etchingbath.

6. The continuous etching process which comprises the steps of providingan etching bath of liquid for contact with a metal article to be etched,continuously feeding to said bath a liquid solution of ferric chloridecontaining a relatively low concentration of chloride of the metal to beetched, and continuously withdrawing from said bath a liquid solution ofa mixture of ferrie chloride and ferrous chloride containing arelatively high concentration of chloride of the metal to be etched,reacting said mixture of ferrie chloride and ferrous chloride withchlorine gas to convert the ferrous chloride therein to ferrie chloride,then cooling to remove a substantial part of the etched metal chlorideby crystallization and thereafter rel0 cycling the liquid solutioncontaining ferrie chloride to said bath.

7. A process according to claim 1 wherein the metal to be etched iscopper.

8. In the process of etching a metal With a ferric chloride etching bathin which process the ferric chloride is reduced to ferrous chloride, theimprovement comprising rejuvenating the spent mixture by converting theferrous chloride to ferric chloride with the aid of chlorine, removing asubstantial proportion of the chloride of the etched metal andthereafter returning the rejuvenated mixture to the etching bath.

9. A process according to claim 8 wherein the metal is copper.

l0. A process according to claim 8 wherein prior to rejuvenation atleast a portion of the spent mixture is passed over iron to replace theiron lost during removal of the etched chloride.

References Cited in the file of this patent UNITED STATES PATENTS665,426 Gibbs Jan. 8, 1901 1,039,586 Patten Sept. 24, 1912 1,312,756Stover Aug. 12, 1919 2,026,603 Zarse Ian. 7, 1936 2,198,412 McDonaldApr. 23, 1940 2,746,848 Jones May 22, 1956 OTHER REFERENCES MetalsHandbook, 1939 ed., pp. 1471 and 2, Etching Copper by C. H. Davis.

1. THE CONTINUOUS PROCESS OF ETCHING METAL WITH FERIC CHLORIDE WHICHCOMPRISES THE STEPS OF BRINGING METAL TO BE ETCHED INTO CONTACT WITH ALIQUID ETCHING BATH THE ACTIVE INGREDIENT OF WHICH IS FERRIC CHLORIDE,TO FORM IN SAID BATH A LIQUID MIXTURE OF FERRIC CHLORIDE, FERROUSCHLORIDE, AND CHLORIDE OF THE ETHED METAL, CONTINUING THE ETCHING UNTILTHE CONCENTRATION OF ETCHED METAL CHLORIDE IN SAID BATH APPROACHES THESATURATION POINT AT THE WORKING TEMPERATURE OF THE BATH, CONTINUOUSLYWITHDRAWING FROM THE BATH A QUANTITY OF SAID LIQUID CONTAINING SAIDCONCENTRATION OF ETCHED METAL CHLORIDE, SUBJECTING THE WITHDRAWN LIQUIDTO REACTION WITH CHLORINE GAS TO CONVERT AT LEAST A SUBSTANTIAL PART OFTHE FERROUS CHLORIDE THEREIN TO FERRIC CHLORIDE, COOLING THE WITHDRAWNLIQUID TO CRYSTALLIZE OUT A SUBSTANTIAL PART OF THE ETCHED METALCHLORIDE, REMOVING THE ETCHED METAL CHLORIDE CRYSTALS FROM THE COOLEDLIQUID, REHEATING THE COOLED LIQUID TO THE WORKING TEMPERATURE OF THEETCHING BATH, AND RETURNING THE REGENERATED LIQUID TO THE ETCHING BATH.